In a nutshell

1. Versioning: keep every change made to files
2. Branching: develop new features while fixing bugs on a production release
3. Decentralized & Collaborative

Outline

1. Git internals
2. 5-min break
3. Common commands
4. 5-min break
5. Tips and tricks for efficient programming

Git internals

What’s Git?

First of all, Git lives in .git/

What’s Git?

Second of all, Git “is” a tree of commits.

What’s a commit then?

• A snapshot of the whole codebase at a given time
• This snapshot is represented as a SHA-1 hash

SHA-1

SHA-1 is an algorithm that takes some data as input and generates a unique 40 character string from it.

The SHA-1 hash depends on…

1. The commit message
2. The committer info
3. The commit date
4. The author info
5. The authoring date
6. The source tree (= the state of the code)
7. The commit’s parent commits

If any of these values change, the SHA-1 changes.

https://blog.thoughtram.io/git/2014/11/18/the-anatomy-of-a-git-commit.html

What’s Git again?

What’s Git again?

1. Git holds a tree of commits

What’s Git again?

1. Git holds a tree of commits
2. Commits have 1 or 2 parents tops

What’s Git again?

1. Git holds a tree of commits
2. Commits have 1 or 2 parents tops
3. There are special cases of orphan commits (no parents) such as the first commit

What’s Git again?

1. Git holds a tree of commits
2. Commits have 1 or 2 parents tops
3. There are special cases of orphan commits (no parents) such as the first commit
4. A branch is a label/tag/pointer/reference (pick the word of your liking) pointing towards a commit

Two types of branches

1. Local writable branches
2. Distant read-only branches (prefixed with origin/)

Special pointers

• HEAD: points to the currently checked out commit
• @: an alias for HEAD

HEAD~ vs HEAD^

www.paulboxley.com/blog/2011/06/git-caret-and-tilde

• ref~ is shorthand for ref~1 and means the commit’s first parent. ref~2 means the commit’s first parent’s first parent. And so on.
• ref^ is shorthand for ref^1 and means the commit’s first parent. But where the two differ is that ref^2 means the commit’s second parent

Let’s look under the hood

Remember: Git lives in .git/

$ cat .git/refs/heads/master
fgjh75955bd59c7ef27d0c55b9165db6a2b768ce

$ cat .git/HEAD
ref: refs/heads/feature-xyz

Everything is a pointer pointing at a commit.

Common commands

How to commit?

Some vocabulary first

• Workspace / working directory
• Index / staging area
• Local repository

git fetch

git pull

= git fetch + git merge

git merge

To fast forward, or not to fast forward…

git rebase

• git rebase origin/master
• git rebase -i origin/master
• git rebase --onto origin/master SHA-1

Rebasing rewrites history.

git diff

• git diff commit1..commit2
• git diff commit1..
• git diff --staged

git show

• git show SHA-1
• git show --stat

git checkout

• Create branch:
  git checkout -b feature origin/master
• Checkout branch:
  git checkout BRANCH
• Checkout files at specific revision:
  git checkout SHA-1 -- file1 file2

git branch

• git branch -vv
• git branch -m new-name

git merge-base

git merge-base origin/master HEAD

==> Common ancestor between origin/master & HEAD

(= between your branch and master)

git revert <SHA-1>

git stash

• git stash
• git stash pop

git log

• git log -p -- package.json
• git log --oneline $(git merge-base @ "origin/master")..
• git log --stat

git reset

• Unstage files:
  git reset file
  
  
• Move branch to other commit:
  git reset --hard SHA-1

git bisect

Finds the commit which introduced a bug.

git reflog

History of all actions performed.

Tips and tricks

for efficient programming

Use the CLI and RTFM

Forget about the GUI

man git-merge

git rebase -i $(git merge-base origin/master HEAD)

When you want to edit/squash some commits and leave other untouched, while keeping most SHA-1 hashes

git lg

github.com/rpellerin/dotfiles/blob/master/.gitconfig

hub pull-request

github.com/github/hub

Delete merged branches

git branch -r | awk '{print $1}' | egrep -v -f /dev/fd/0 <(git branch -vv | grep origin) | awk '{print $1}' | xargs git branch -D

Open last modified files

https://github.com/rpellerin/dotfiles/blob/master/.aliases

Built-in aliases for common Git commands

alias | grep git

(go ahead, run the command right now)

That’s it! Thanks.